6. SUMMARY

There are two broad, interrelated questions concerning star formation
in normal, non-interacting
irregular galaxies whose answers would take us a long ways towards
understanding global star formation processes and galaxy evolution.
The first is ``What governs
the overall rate at which a galaxy forms stars?'' Irregulars having
very similar global properties nevertheless show a wide range in
total star formation rates relative to their sizes. Secondly,
``What initiates star/cloud formation within irregular galaxies?''
Since spiral density waves, which were once believed to be the primary
triggers for star formation, are not applicable in irregular
galaxies, we must determine what processes do control star
formation in these systems. These processes may also operate
in spiral galaxies but be difficult to disentangle from the effects
of the more dominant density wave.
Thus, we need to learn what regulates star formation locally in irregular
galaxies,
what feedback mechanisms are operating and their relative
importance, what consequences there are to the star formation
process due to differences in the
ISMs of irregulars compared to spirals, and what role the extended
gas plays in the evolution of the optical irregular galaxy.

We find that star formation is a local or regional process in the sense that
it is radially similar today to what it has been historically over
large time-scales. Of course, star formation in small galaxies
is a ``grainy'' process and there are stochastic variations on
short time-scales, but integrated over billions of years star
formation is regionally constant.
In the context of a simple thin rotating disk model, the gas in irregulars
appears to be more stable against cloud formation than in spirals,
but none of the models in the literature correctly predicts the radial
star formation pattern in irregulars. This suggests that the models
are too simple and that other factors, such as feedback from
massive stars, random gas motions, and lack of shear may play a larger role
in facilitating star formation in irregulars than they may play in spirals.
The signatures of feedback from concentrations of massive stars
are frequently seen in the ISMs of irregulars and in some cases
clear examples of star-induced star formation are seen as well. However,
the collective global contribution of this feedback process to
star formation in irregulars is not clear, although arguments
have been made that the net global effect must be to inhibit further
star formation.

In spite of differences between irregulars and spirals in terms
of their ISMs, the stellar products of the star formation process
and the efficiencies of turning a cloud into stars appear to be the same.
Thus, the local process of star formation, what happens inside a cloud
once the cloud has formed, is not a function of the type of galaxy.
One consequence to differences in amounts of interstellar shear, however,
is that irregulars can form larger gas clouds which collapse quickly
to form giant H II regions. These concentrations in time and space
of large numbers of massive stars will have a larger impact on
the ISM than the same number of stars formed over a larger area
and time period.

Most irregular galaxies have H I gas that is extended well-beyond the stars,
and some have gas that is unusually extended - up to seven times the
Holmberg radius. This far-flung gas is potentially a vast reservoir
from which to fuel star formation for a very long time, but there
is no evidence at present that these H I envelopes are having a major
impact on the optical galaxy. However, we cannot rule out a very slow
replenishment of the gas in the center of the galaxy as star
formation depletes the gas there. A few of the unusually extended gas
envelopes are now known to be highly structured, inhomogeneous,
and not quiescent, and it is probable that those systems will change
radically with time.

The issues of star formation and the ISM of irregular galaxies are
related to other areas of astronomical research. Irregular galaxies
are particularly underevolved compared to spiral and elliptical
galaxies and so they pose an interesting glimpse of the early
stages of galaxy evolution. Furthermore, their large, extended
gas envelopes offer a chance to examine the role that such components
of the galaxy may play in the development of the optical galaxy,
and the extended gas may be connected to QSO absorption
line systems. In addition dwarf spheroidals, such as those that
are companions to the Milky Way, are believed to be
fossils of the smallest irregular galaxies.
Finally, the class of irregular galaxies referred to as Blue Compact
Dwarfs, not discussed here,
are considered to be candidates for the faint blue galaxy
population seen at large look-back times and can possibly
shed some insight into the galactic starburst phenomenum believed to
account for the faint blue galaxies.
Thus, irregular galaxies are a rich mine for furthering our understanding
of galactic processes.

ACKNOWLEDGMENTS

I would like to thank the Instituto de Astrofísica de Canarias
for hospitality while this paper was being worked on.
I am very grateful to R. Kennicutt, R. Larson, and P. Massey for
comments on a draft of this paper.